(1) Field of the Invention
This invention relates generally to fuel injection systems for internal combustion engines and, more particularly, to electronically-operated piezoelectric control valves making use of a piezoelectric effect for regulating injection timing and quantity of fuel.
(2) Description of the Prior Art
It is well known in the art that electric control of fuel injection systems enables flexible regulation of injection timing and quantity in accordance with various engine parameters. In this regard, the application of a piezoelectric effect to injection control valves has been proven to provide a most desirable response to electrical injection pulses because piezoelectric elements undergo piezoelectric deformation almost instantaneously when subjected to an electric field. Known piezoelectric control valves for controlling internal combustion engine fuel injection systems generally comprise a column of piezoelectric discs coupled to a shut-off valve member. A piezoelectric control valve is placed between a source of high-pressure fuel and an injector of the differential pressure type to on/off control the flow of fuel flowing into the injector. However, the amount of piezoelectric deformation of the piezoelectric elements available for displacement of the shut-off valve member is very small, for example, on the order of a fraction of a millimeter, so that even the slightest longitudinal expansion or contraction of the valve housing and piezoelectric elements due to a variation in the ambient temperature would hinder proper performance of the control valve.
Moreover, conventional piezoelectric hydraulic control valves are designed merely to on/off control the flow of fuel. This means that although a rapid pressure build-up is obtained at the injector upon energization of a valve, the pressure, once it is built-up in the fuel supply line between the valve and the injector nozzle, remains the same even after the valve is de-energized. The fuel pressure prevailing at the injection nozzle hinders precise control of the injection period and, hence, quantity control.
The above-mentioned impossibility of obtaining a pressure collapse becomes critical in fuel injection systems having a servomultiplier wherein the outlet port of a piezoelectric control valve is connected to a pilot-operated spool valve which is driven by a pilot pressure signal from the piezoelectric control valve to operate the servomultiplier, which multiplies the supply pressure to feed ultra high-pressure fuel to the injectors. In this instance, if the pressure of the pilot signal applied from the piezoelectric control valve to a pilot port of the spool valve remains the same when the driving pulse is terminated, the spool valve cannot perform a cyclic movement.